The purpose of this study is to determine in patients with malignant solid tumors the qualitative and quantitative toxicities of 9-Amino-20(S)- Camptothecin (9-AC) when given by prolonged low-dose infusion and its maximally tolerated dose; to determine the pharmacokinetics and steady state levels achieved using this schedule; to study treatment correlates in available tumor tissues; and to document anti-tumor activity observed using this schedule.